1. Field of the Invention
This invention relates to an improved process for recovery of hydrocarbons from a porous reservoir. More particularly, the invention relates to an improved enhanced oil recovery process for recovery of hydrocarbons from a porous reservoir which involves injecting a material into the reservoir.
2. Background
A large portion of the original oil in place in many oil-bearing subterranean formations remains in place after primary production and water flooding. As oil reserves dwindle and exploration for new discoveries becomes more difficult and costly, the use of enhanced oil recovery techniques on previously discovered resources will play an increasingly important role in the overall production of crude petroleum.
Because of the porous nature of oil-bearing formations or reservoirs, the formation itself exerts capillary forces on the contained oil. Viscous forces are also exerted on the oil in the formation. Studies have shown that, for immiscible displacement EOR techniques such as waterflooding, as the ratio of capillary to viscous forces decreases the fraction of oil recovered increases.
There are three generic types of enhanced oil recovery (EOR) processing generally recognized in the industry. The first of these is thermal processing, e.g., steam soak, steam drive, in-situ combustion, to reduce the viscosity of heavy, highly viscous oils, and thereby increase oil recovery.
A second EOR technique is miscible flooding. Various materials miscible in the residual crude oil have been suggested for injection into the porous formation or reservoir in an attempt to increase the production of crude oil. Such materials have included liquified petroleum gas (LPG), propane and carbon dioxide. For miscible flooding processes, the ratio of capillary to viscous forces is zero and 100% residual oil recovery is theoretically possible. However, miscible flooding is a relatively expensive and high risk EOR technique. For example, the relative price of LPG and propane to crude petroleum may make the use of these solvents economically prohibitive. Substantial capital may be required to produce carbon dioxide and/or transport carbon dioxide to the well site. Also, the amount of oil that can be recovered using these miscible flooding techniques is dependent on the type of crude petroleum to be recovered and even on the configuration and condition of the individual porous reservoir involved. Miscible flooding may work extremely well in one situation and have no substantial effect in another instance. Hence, miscible flooding is a high risk EOR technique.
A third EOR processing option involves the use of micellar/polymer fluids. These fluids are typically aqueous solutions which contain surfactants at relatively high concentrations above the critical micellar value and polymers, such as polysaccharides and hydrolyzed polyacrylamides, that develop aqueous phase viscosities that provide stable displacement with reduced bypassing. For micellar processes, the interfacial forces are reduced sufficiently to approach a miscible-like displacement, i.e., oil recoveries approaching 100% are theoretically possible. Field experience with the technique has been generally disappointing. This performance can be traced back to the fact that because of the cost of these systems, the micellar/polymer fluids can be injected only in slugs, rather than continuously, if economic recovery of crude oil is to be achieved. The slug size is generally limited to less than 5-10% of the reservoir pore volume. Also, the integrity of the slug is weakened by numerous factors, such as temperature and shear degradation, precipitation by ions occurring in the connate water or released by ion exchange with the reservoir clays, adsorption on mineral surfaces, cross-flow and diffusion into low permeability layers, and transfer of the active surfactants into the oil phase. In short, the relatively expensive micellar/polymer fluids are often not cost effective as EOR agents.
The use of surfactants during ordinary waterflooding decreases the capillary to viscous forces ratio to some extent, however, generally not enough to substantially increase oil recovery. In any event, an improved enhanced oil recovery process would clearly be advantageous.